Heart rate may be measured using any of a variety of different sensors, including, for example, electrode-based sensors, such as EKG sensors, and optical sensors, such as photoplethysmographic (PPG) sensors. PPG sensors typically include a light source and a photodetector that are placed adjacent to a person's skin. The light source and photodetector are typically arranged so that light from the light source cannot reach the photodetector directly. However, when the PPG sensor is placed adjacent to a person's skin, light from the light source may diffuse into the person's flesh and then be emitted back out of the person's flesh such that the photodetector can detect it. The amount of such light that is emitted from the person's flesh may vary as a function of heart rate, since the amount of blood present in the flesh varies as a function of heart rate and the amount of light that is emitted from the person's flesh, in turn, varies as a function of the amount of blood present.
The assignee of this application, Fitbit, Inc., makes wearable fitness monitoring devices, some of which, such as the Charge HR™ and the Surge™, incorporate PPG sensors that include two high-brightness, green light-emitting diodes (LEDs) that are spaced approximately 8 mm apart, as well as a 2 mm square photodetector element located midway between the LEDs. Various other companies that make wearable fitness monitoring devices utilize a similar architecture. For example, the Basis Peak™ incorporates two green LEDs with a square photodetector element located midway between them, as does the Motorola Moto 360™.
FIG. 1 depicts a simplified representation of a prior-art wristband-type wearable fitness monitor 100 that incorporates a PPG sensor. The wearable fitness monitor 100, in this example, includes a housing 104 with two straps 102 attached; the straps 102 may be used to fasten the housing 104 to a person's forearm, in much the same manner as a watch (indeed, many such devices may incorporate timekeeping functionality as well). The PPG sensor, in this example, includes two light sources 108, with a photodetector element 112 interposed midway between them on a back face 128 of the housing 104; the photodetector element 112 in this example has a photosensitive area with a square aspect ratio. When the wearable fitness monitor 100 is worn by a person in a manner similar to a wristwatch, the back face 128 may be pressed against the person's skin, allowing the light sources 108 to illuminate the person's skin. The photodetector element 112 may then measure the amount of that light that is then emanated back out of the person's skin. Control logic (not pictured) within the housing 104 may then calculate the person's heart rate based on fluctuations in the amount of light measured by the photodetector element 112.